Selective Breeding
Use of Biological Resources · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 9% of your exam marks.
Selective breeding vs natural selection comparisons and examples appear across most papers.
(also called micropropagation) is a modern technique for rapidly cloning plants by growing many small pieces of plant tissue in sterile laboratory conditions. It is an alternative to traditional methods like taking cuttings (covered in topic 12).
How tissue culture works
- Select a parent plant with the desired characteristics (high yield, disease resistance, attractive flowers).
- Take small (small pieces of tissue) from a fast-growing region of the plant, usually a shoot tip or meristem.
- Sterilise the explants by washing in dilute bleach to kill any microbes on the surface.
- Place the explants on a sterile nutrient medium (agar containing sugars, mineral ions, and plant hormones such as auxin and cytokinin) in a sealed Petri dish or culture vessel.
- The cells divide and form a callus (a mass of unspecialised cells) under the influence of the hormones.
- Change the hormone balance in the medium to encourage the callus to develop into tiny plantlets (shoots and roots).
- As soon as small plantlets have formed, transfer them to soil and grow them on like ordinary plants.
The entire process can take just a few weeks. A single parent plant can produce thousands of identical offspring in one year, all genetically identical to the parent and to each other.
Advantages of tissue culture
- Very fast: thousands of clones from one parent in months, not years
- Genetically identical offspring, so all share the parent's desirable traits
- Disease-free: the sterile starting conditions and small explants mean the new plants are free of viruses and other pathogens that the parent might have been carrying
- Year-round production, not dependent on natural growing seasons
- Small space: many plantlets can be grown in a single laboratory
- Useful for endangered plant species where there are too few individuals left to propagate by traditional methods
Disadvantages
- Expensive: needs sterile equipment, trained staff, and specific growing media
- All plants are genetically identical, with the same reduced gene pool problem as in selective breeding. A disease that affects one will affect all
- Some plant species do not respond well to tissue culture
- The plantlets are delicate during transfer to soil; many die at this stage
Describing the tissue culture process
What comes up: "Describe the process of micropropagation / tissue culture." Typically 3 marks, awarding one mark per correctly described step.
Write (three marks): (1) Take small pieces of tissue (e.g. shoot tips or small cell fragments) from the parent plant. (2) Sterilise the tissue samples (using alcohol, disinfectant, or bleach) to kill any microorganisms, then place them into sterile agar. (3) Supply the growing medium with minerals, sugars, and plant growth substances (such as auxin) so that the cells divide and develop into new plantlets.
Watch out: Stating "sterile agar" alone scores two marks in one step (sterilisation + agar placement). If you write only "place into agar" without mentioning sterilisation, you get just one of those two. Writing "nutrients" alone without naming a type (minerals, glucose, amino acids) is credited, but naming a specific type (e.g. glucose) is the safest option.
Uses in practice
Tissue culture is widely used for:
- Mass production of commercially valuable plants (orchids, banana, oil palm, sugar cane, potato)
- Disease elimination in seed potato production (small pieces of meristem from infected plants are free of virus and grow into virus-free plants)
- Plant breeding programmes, where new varieties can be quickly multiplied for testing
- Conservation of rare and endangered species
- Genetic engineering (covered in topic 18), where modified cells need to be grown into whole plants